The present invention relates to a device for removing a web from a drying cylinder, preferably in a dry end of a machine for the manufacture or treatment of a web of paper, board or the like, and particularly to a device which applies suction for effecting removal of the web from the drying cylinder, and is designed to prevent damage in the event of a disturbance during the web travel, such as the presence of a wad.
The drying section of a paper making machine typically includes at least one and usually a plurality of drying cylinders arranged in sequence of the path of the web. Typically, there is a web support belt that passes the web around successive dryer cylinders. In some arrangements, there is a generally unheated support belt guiding, reversing roll disposed between neighboring drying cylinders in the path of the web through the drying section. The support belt partially wraps around each of the drying cylinders. The web is supported on the side of the support belt so that the web directly contacts the drying cylinders, while the web travels on the opposite outer side of the support belt around the reversing rolls. Typically, the reversing rolls are suction rolls.
The web should leave the drying cylinder with the support belt. As described below, appropriate suction means are provided for accomplishing that result.
The starting point for the invention is German DE-OS 39 10 600 which corresponds to International Application WO 90/12151. That reference mentions the problem that the paper web has a tendency to travel initially some distance further with the outer surface of a drying cylinder beyond the desired point of web removal from the drying cylinder and only thereafter does the web again come against the support belt. Such a course of movement of the web is undesirable, because it leads to unstable web behavior and/or because the web is stretched in the length direction at its point of removal from the drying cylinder. This accordingly causes the web to shrink in the transverse direction. In order to avoid these disadvantages, DE-OS 39 10 600 suggests producing a vacuum on the inner side of the support belt in the region where the support belt travels off the cylinder, at the suction box 20 with suction openings 26 and 26' in that reference. Furthermore, a sealing strip is provided which extends transversely over the inner side of the wire support belt shortly in front of the point of removal. This strip deflects the air boundary layer which arrives together with the support belt, since otherwise the vacuum could not be produced.
Another known device attempts to draw the web of paper against the support belt as close as possible to the point of removal of the support belt from the drying cylinder comprises a so-called web stabilizer. This is shown in Federal Republic of Germany Patent 37 06 452, which corresponds to U.S. Pat. No. 4,856,205.
In both of these known devices, the vacuum produced at the point where the support belt travels off the drying cylinder is not always sufficient to assure dependable guidance of the web of paper. Furthermore, in rare cases, a foreign body, for instance, a wad of fibers, is brought, together with the web of paper, between the drying cylinder and the support belt. It becomes jammed there at the strip. This presents a danger of the support belt being destroyed. A similar disturbance occurs if the drying cylinder becomes "wrapped-up" in the event of a tear in the web, i.e., if the web winds up in an undesired manner on the drying cylinder.
In order to avoid these dangers, which may result from the above noted disturbances, U.S. Pat. No. 4,359,828, FIGS. 4 to 6, proposes to make a sealing strip flexible so that it can move away from a wad which arrives together with the web. Furthermore, in that case, a part of the front wall of a suction box is mounted movably and is pressed against the support belt under the force of a spring. This enables this part of the wall to also move out from a foreign body. In FIG. 3 of U.S. Pat. No. 4,359,828, a suction zone for a relatively high vacuum is defined, near the place where the support belt runs off the cylinder, by two yieldable sealing strips which extend transversely to the direction of travel of the web. Each sealing strip is supported in a respective sealing strip holder.